1. Field of the Invention
The present invention relates to uniformity correction modules, lithographic apparatus comprising a uniformity correction module, a method of increasing the uniformity of an illumination beam, and a device manufacturing method using a uniformity correction module.
2. Background of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The invention relates to uniformity correction modules (sometimes referred to as “unicoms”) that consist of a plurality of light-absorbing elements the positions of which are adjustable in the scanning direction in order to set the outer boundary of the illumination slit. Such a unicom (sometimes also called dynamic adjustable slit or “DYAS”) is described in U.S. Pat. No. 6,097,474. The elements may for example be embodied as transmissive quartz plates which are coated with a semi-transparent layer. A disadvantage of these plates is that refraction occurs at the air-quartz interface at the edges leading to ellipticity, telecentricity, local straylight or hotspot errors.
Current uniformity correction modules function in air or vacuum i.e. in an ambient with a refractive index n=1. For the coming generation high numerical aperture (NA) systems (NA>0.5) no feasible solution exists to use a known concept that does not affect the pupil and telecentricity distribution negatively. The pupil distribution is the intensity distribution in the pupil plane of the projection system, and corresponds with the angular intensity distribution of the projection beam. The telecentricity of a radiation beam impinging on a point on a wafer is the average incident angle. Preferably the telecentricity is perpendicular to the wafer surface but in practice it shows a slight variation over the illuminated field/slit.